Field of the Invention
This invention relates to systems and methods for configuring operating conditions for at least one of desalination equipment and fluid injection equipment to be used in a low salinity waterflood on a hydrocarbon-bearing reservoir. In particular this invention relates to systems and methods to be used when the reservoir comprises relatively permeable layers interbedded with relatively impermeable layers and where the relatively impermeable layers have a relatively high concentration of ions compared to that of the relatively permeable layers when the low salinity water is present therein.
Background of the Invention
A hydrocarbon-bearing reservoir typically takes the form of a plurality of sandstone layers interbedded with shale layers. The sandstone layers have sufficient porosity and permeability to store and transmit fluids (for example oil and water). Typically the oil is held in pores of the rock formation. By contrast the shale layers are relatively impermeable to these fluids.
It is known that only a portion of the total crude oil present in a reservoir can be recovered during a primary recovery process, this primary process resulting in oil being recovered under the natural energy of the reservoir. Secondary recovery techniques are therefore often used to force additional oil out of the reservoir. One example of a secondary recovery technique is to directly replace the oil with a displacement fluid (also referred to as an injection fluid), usually water or gas.
Enhanced oil recovery (EOR) techniques may also be used. The purpose of such EOR techniques is not only to restore or maintain reservoir pressure (as is done by typical secondary recovery techniques), but also to improve the displacement of the oil from the reservoir, thereby maximizing the recovery of oil from the reservoir and minimizing the residual oil saturation of the reservoir (the volume of oil present in the reservoir).
“Waterflooding” is one of the most successful and extensively used secondary recovery methods. Water is injected, under pressure, into reservoir rock layers via injection wells. The injected water acts to help maintain reservoir pressure, and sweeps the displaced oil ahead of it through the rock towards production wells from which the oil is recovered. The water used in waterflooding is generally saline water from a natural source (such as seawater) or may be produced water (i.e. water that is separated from the crude oil at a production facility).
In addition to waterflooding using saline water, it is possible to use lower salinity injection water (for example, brackish water such as estuarine water, or fresh water such as river water, or lake water). The use of low salinity waterflooding can increase the amount of oil recovered compared to that recovered using high salinity water since the low salinity water is better able to displace the oil from the reservoir.
The water used in a low salinity waterflood typically has a total dissolved solids (TDS) content in the range of 500 to 12,000 ppm. It is also preferred that the ratio of the total multivalent cation content of the low salinity injection water to the multivalent cation content of the formation water that is present in the sandstone layers of the reservoir is less than 1. The use of a low salinity waterflood is particularly beneficial when oil that is present in the sandstone layers of the reservoir (typically oil that is adhering to the surface of the sandstone rock) is a medium or light crude having an American Petroleum Institute (API) gravity of at least 15° C., preferably at least 20° C., and for example an API gravity in the range of 20° C. to 60° C.
During a low salinity waterflood, the low salinity injection water is injected into and flows through the sandstone layers of the reservoir. By contrast, little water flows through the relatively impermeable shale layers. Thus, the oil is produced from the high permeability sandstone layers while insignificant amounts of oil are produced from the low permeability shale layers. Indeed, shale is often so impermeable that the interbedded shale layers of the reservoir remain unsaturated with oil during migration of oil from a source rock into the sandstone layers of the reservoir. Instead, the shale layers are saturated with connate water that is typically of high salinity.
It has now been found that for reservoirs having interbedded sandstone and shale layers, the incremental oil recovery effect that is achieved using low salinity waterflooding may be reduced. This is due to the diffusion of ions from higher salinity connate water present in the pore space of the shale layers into the low salinity water that is flowing through the adjacent sandstone layer of the reservoir. This reduction in recovery is of particular concern when large volumes of high salinity connate water reside in shale layers that are interbedded with the sandstone layers of the reservoir and when the interbedded sandstone layers are relatively thin.
The dominant mass transfer mechanism from the connate water of the shale layers to low salinity water that is flowing through the adjacent sandstone layers of a reservoir is molecular diffusion, whereby salt ions diffuse from the connate water in the shale layer to the low salinity water in the sandstone layer. Typically, the molecular diffusion of salt ions from the shale layer occurs in a direction substantially orthogonal to the direction of flow of the low salinity water through the adjacent sandstone layer (i.e. in the direction of the concentration gradient).
The diffusion of the salt ions from higher salinity connate water present in the pore space of the shale layers can reduce the effectiveness of a low salinity waterflood by increasing the salinity of the water flowing through the sandstone layers. It is therefore an object of the invention to determine the effectiveness of low salinity waterflooding and to consequently configure operating conditions of desalination equipment and/or fluid injection equipment to be used in the low salinity waterflooding.